The pharmacologic treatment of children suffering from the particular combination of attention deficit hyperactivity disorder (ADHD) and chronic tics has been problematic. Although stimulants can be very effective for ADHD, such drugs are known to accentuate tics in some patients. Recent studies indicate that methylphenidate (MPH) may be the stimulant least likely to exacerbate tics and that such effects may be minor or temporary. Many open-label reports indicate that clonidine is beneficial, but recent controlled clinical trials have produced conflicting results. No study has examined the combined effects of MPH and clonidine. We propose a randomized, double-blind, parallel groups clinical trial which employs a 2 x 2 factorial design, whereby children with ADHD and chronic tics are assigned to one of 4 treatments: 1) MPH, 2) clonidine, 3) MPH plus clonidine, and 4) placebo. The study will involve 136 subjects enrolled among 7 centers. Each subject will participate in the trial for 16 weeks. A teacher-derived measure of ADHD will serve as the primary endpoint variable. Secondary measures of ADHD will include a parent-derived rating, another teacher rating, a computerized performance task and independent ratings of observed classroom behavior. Secondary analyses will also assess effects on tic severity and ratings of clinical improvement and global functioning. Since recent evidence indicates that ADHD and tics commonly coexist, the determination of the optimal treatment for this patient population is particularly important.